1. Field of the Invention
The present invention relates to a multi-domain liquid crystal display device.
2. Description of the Related Art
In a lateral electric field liquid crystal display device, as illustrated in FIG. 8, regions surrounded by drain lines DL and gate lines GL correspond to regions of pixels PXL, and a pixel electrode PX is formed in each of the pixels PXL. In the lateral electric field liquid crystal display device having such a structure, a common electrode is formed in an upper layer of the pixel electrodes PX through intermediation of an insulating film. In a region in which the common electrode overlaps the pixel electrode PX, slits SLT are formed, which are opened in a Y-direction indicated by arrows Y1 and Y2. The linear common electrodes are arranged so as to overlap the flat-plate like pixel electrode PX through intermediation of the insulating film. Here, in a case of a multi-domain liquid crystal display device, in each pixel PXL region, the slits SLT on the Y1-side and the slits SLT on the Y2-side have different inclined angles with respect to the Y-direction. With this structure, as indicated by arrows R1 and R2 of FIG. 8, it is possible to rotate liquid crystal molecules on the Y1-side of the pixel PXL and liquid crystal molecules on the Y2-side thereof in different rotational directions. In this manner, it is possible to improve viewing angle characteristics of the liquid crystal display device.
However, in such a multi-domain liquid crystal display device, it is known that disturbance of an electric field occurs at a boundary region BA at which the inclined angle of the slit SLT changes within the same pixel PXL region, which disturbs normal image display. Therefore, a method of suppressing such disturbance has been desired. As a liquid crystal display device capable of suppressing such disturbance of image display at the boundary region BA, there is a fringe field switching (FFS) liquid crystal display device described in Japanese Patent Application Laid-open No. 2009-181092. In the technology described in Japanese Patent Application Laid-open No. 2009-181092, the FFS liquid crystal display device has a structure in which connection portions of slits formed in upper electrodes (pixel electrodes), the slits being inclined in a plurality of different directions, overlap an opening portion of a lower electrode (common electrode).
Further, as a structure capable of realizing the multi-domain structure in a lateral electric field liquid crystal display device of an in-plane switching (IPS) type, there are technologies described in Japanese Patent Application Laid-open Nos. 2003-215636 and 2007-199745. In the technology described in Japanese Patent Application Laid-open No. 2003-215636, a liquid crystal display device with delta layout has a structure in which pixels curved in directions opposite to each other are arranged. Further, in the technology described in Japanese Patent Application Laid-open No. 2007-199745, a pixel having sides with angles opposite to each other is provided, to thereby improve uniformity of color tone.
In addition, as a structure capable of realizing the multi-domain structure in the FFS liquid crystal display device, there is a technology described in Japanese Patent Application Laid-open No. 2008-276172. In the technology described in Japanese Patent Application Laid-open No. 2008-276172, there are provided continuous portions corresponding to linear slits of a common electrode formed across adjacent pixels, and the slits have inclined angles which are in symmetry with respect to a boundary of unit pixels for color display of red (R), green (G), and blue (B). With this structure, it is possible to reduce viewing angle dependency.
In recent years, as for the liquid crystal display device to be mounted onto a mobile terminal device such as a mobile phone, although the size of a casing is limited, there have been demands for high definition and high image quality. In order to obtain high definition, it is necessary to form a large number of pixels within a limited display region, and hence an area occupied by one pixel is reduced. However, in order to form a thin film transistor for driving a pixel, a storage capacitor, and the like, a predetermined area is necessary, and hence an aperture ratio, which corresponds to an area of a region which transmits backlight with respect to an area occupied by the pixel, is decreased. Therefore, improvement in aperture ratio has been desired.
On the other hand, in order to improve display quality and achieve high image quality, it is necessary to improve brightness of the display image, and hence, similarly to the case of high definition, improvement in aperture ratio has been desired. In order to improve display brightness, it is conceivable to employ a method of increasing a light intensity of the backlight, but as for the mobile terminal device, there is also a demand for long battery life. Further, as for an image display device, there is a demand for reduction in power consumption of a backlight device, which has relatively large power consumption. Therefore, it is extremely difficult to increase the light intensity of the backlight.
Ina liquid crystal display device described in Japanese Patent Application Laid-open No. 2009-181092, the opening portion is formed in the lower electrode (common electrode), the opening portion overlapping boundary regions of the slits, which correspond to the connection portions of the slits and at which the inclined angle of the slit changes. In the region in which the opening portion is formed, an electric field, which rotates and drives the liquid crystal molecules, is not generated. With such a structure, the liquid crystal molecules in this region are held in the initial orientation state, and thus a black display region which does not transmit backlight is generated. Further, by causing the liquid crystal molecules in this region to be held in the initial orientation state, when the liquid crystal display device is pressed from an image display surface thereof, expansion or movement of a region which is incapable of normal image display is prevented. In the liquid crystal display device described in Japanese Patent Application Laid-open No. 2009-181092, black display is performed at the connection portions of the slits which overlap with the opening portion, and hence the connection portions do not contribute to image display. Therefore, there is a fear that the aperture ratio of the pixel decreases.
Further, in the technology described in Japanese Patent Application Laid-open No. 2008-276172, in one pixel region (sub-pixel region) surrounded by the drain lines and the gate lines, the inclined angles of the slits are all in the same direction, and the inclined angles of the slits are formed in different directions for respective adjacent pixel regions. Because of this structure, the one pixel (sub-pixel) has the viewing angle dependency, and there is a fear that the effect of reducing the viewing angle dependency by the multi-domain structure is lowered.
Meanwhile, the technology described in Japanese Patent Application Laid-open No. 2003-215636 has an object of suppressing breakage of the drain signal lines, and the technology described in Japanese Patent Application Laid-open No. 2007-199745 has an object of improving uniformity of the color tone of the display image. Yet, neither consideration nor description is made about disturbance of the image display due to the disturbance of the electric field generated at the curved portion of the pixel electrode.